With recent predictions about the exhaustion of existing energy resources such as petroleum and coal, interests in energy capable of replacing these have been growing. As one of such alternative energy, fuel cells have received particular attention with advantages of being highly efficient, not emitting pollutants such as NOx and SOx, and having sufficient fuel to use.
Fuel cells are a power generating system converting chemical reaction energy of fuel and oxidizer to electric energy, and hydrogen, methanol and hydrocarbon such as butane are used as the fuel, and oxygen is typically used as the oxidizer.
Fuel cells include polymer electrolyte membrane-type fuel cells (PEMFC), direct methanol-type fuel cells (DMFC), phosphoric acid-type fuel cells (PAFC), alkaline-type fuel cells (AFC), molten carbonate-type fuel cells (MCFC), solid oxide-type fuel cells (SOFC) and the like.
Among these, solid oxide fuel cells are based on low activated polarization and thereby have low overvoltage, and have small irreversible loss, and accordingly, have high power generation efficiency. In addition, carbon or hydrocarbon-based materials may be used as fuel as well as hydrogen leading to a wide fuel choice, and high-priced precious metals are not required as an electrode catalyst since reaction rates in electrodes are high. Besides, temperatures of heat released incidental to the power generation are very high, which is highly useful. In other words, heat generated in a solid oxide fuel cell may be used not only in fuel reformation, but also as an energy source for industry or cooling in a cogeneration system.
When examining a basic operation principle of such a solid oxide fuel cell (SOFC), a solid oxide fuel cell is basically a device generating power through an oxidation reaction of hydrogen, and an electrode reaction as in the following Reaction Formula 1 is progressed in an anode that is a fuel electrode and a cathode that is an air electrode.Air electrode: (½)O2+2e−→O2−Fuel electrode: H2+O2−→H2O+2e−Whole Reaction: H2+(½)O2→H2O  [Reaction Formula 1]
In other words, electrons reach an air electrode through an external circuit, and at the same time, oxygen ions generated in the air electrode are transferred to a fuel electrode through an electrolyte, and in the fuel electrode, hydrogen and the oxygen ions bond to produce electrons and water.
Meanwhile, a solid oxide fuel cell is formed with a unit cell including an air electrode, an electrolyte and a fuel electrode, and a stack is formed by laminating a number of these unit cells. For such lamination, an air electrode in one unit cell and a fuel electrode in another unit cell need to be electrically connected, and a structure capable of supplying fuel and air to each unit cell is required, and for this, a separator made of a metal is used. Herein, in this fuel cell stack, sealing between the metal separator and constituents of the unit cell is important in order to prevent mixing of hydrogen, a fuel gas, and air, a combustion gas, to prevent a gas leak outside the stack, and for insulation between the unit cells.
In other words, such fuel gas and air need to move through a fixed path, and when the fuel gas and the air are mixed or leaked outside, battery performance rapidly declines, and therefore, a high level of sealing technology is required.